1. Field of the Invention
The present application relates to methods, systems and apparatuses to torsionally stabilize a spinal motion segment.
2. Description of the Related Art
In many patients, an early finding associated with back pain is a weakening or disruption of the annulus. Patients in this state may then be treated with either micro- or open discectomy to remove any fragments associated with pain. Typically, these patients do well in the short term, but eventually have degeneration leading to axial (back or neck) pain, sometimes also in the presence of radicular pain, radicular weakness or a loss of sensation radicularly.
In patients with low back pain generally, including those without disruption of the annulus, there is known to be excessive axial rotation, as recently shown by Haughton et al., Measuring the Axial Rotation of Lumbar Vertebrae in Vivo with MR, Am J Neuroradiol 23: 1110-1116, August 2002. Also, scoliosis patients are known to have changes in the multifidus, which is a significant contributor to spinal stabilization and is a significant generator of axial rotation. In both the population of patients with low back pain, and in scoliosis patients, there may be benefit to a device that increases the stability of the segment(s).
Mechanically, the annulus is a significant structure. In the lumbar spine, the annulus is reported to be on the order of 10 mm thick in the anterior half of the body, but perhaps less than 5 mm posteriorly. As such, it can represent 40 to 60% of the overall area of the endplate. It is known to resist compression, tension, flexion/extension, lateral bending and axial rotation.
With weakening or disruption of the annulus, mechanical changes in the annulus' behavior are expected. It is of value to consider the mechanical impact of annular defects in the different loading directions.
The compression and tension behavior of the annulus is determined by the material properties of the annulus and the annulus' cross-sectional area. The size of the annular defect is a relatively small percentage of the overall annulus. For example, a 10 mm diameter defect in an annulus only represents 8% of the overall annular area. As such, an annular defect has a modest impact on the area, and therefore, the compressive and tensile load carrying capacity of the motion segment.
In flexion/extension or lateral bending motions, the structural behavior of the motion segment is related to the moment of inertia of the annulus. Like tension/compression, the annulus is a significant contributor, and the effect of a defect has a relatively modest impact. Calculations show that an annular defect reduces the moment of inertia of the annulus by only 10%.
In torsion, the structural behavior of the motion segment is related to the polar moment of inertia of the annulus. Using an approximation of a hollow circular cylinder for the annulus, the impact of a hole in the annulus reduces the polar moment of inertia on the order of 90%, and greatly influences the torsional stiffness of the spine. It is therefore desirable to provide systems, methods and apparatuses that may effectively help stiffen motion segment(s) torsionally.